George Zimmerman shot Trayvon in order to defend himself. If he had not shot the gun Trayvon would have, without a doubt, killed him. He was smashing Zimmerman's head against the concrete and one more smash could have easily killed Zimmerman. It is also said that Trayvon was at one point reaching for Zimmerman's gun to shoot him. Clearly, Zimmerman was afraid for his life and he would not be alive today if he had not pulled the trigger.
No he should have to face a trial and be tried as guilty just as everyone else would be if they committed so many crimes in the world. He has been in trouble more then three times and there is a three strike system, that if he was found guilty the first time he would have not been able to commit these other crimes, or would have faced Americas three strike policy.